The present invention relates to a transparent, electric insulating, low-melting glass for covering a substrate, particularly for covering a transparent electrode pattern or bus electrode pattern disposed on a plasma display panel (PDP) substrate. This glass serves as an electric insulating film of a PDP substrate.
Hitherto, lead-containing glass has been used as a low-melting glass for covering a substrate. Lead is an important component for making the glass have a low melting point. However, there is a recent trend to avoid the use of lead due to its adverse effect on human and environment. Furthermore, it is known to introduce an alkali metal(s) and/or bismuth component to a glass batch in order to produce a low-melting glass. However, the alkali metals tend to accumulate on the glass surface, thereby lowering electric insulation of the glass. Thus, there is a tendency to avoid such low-melting glass containing alkali metal in the field of electronics. For example, such low-melting glass may deteriorate a fluorescent material of a display panel (e.g., PDP) and may make the panel lifetime shorter. The above-mentioned bismuth component also may have an adverse effect on environment since bismuth is a heavy metal.
Japanese Patent Examined Publication JP-B-51-17027 discloses a glass for protecting a semiconductor device. This glass contains three major components of SiO2, B2O3 and ZnO, but does not contain BaO.
Japanese Patent Laid-open Publication JP-A-59-131540 discloses a glass composition for making an insulating film. This glass composition contains 0.05-3.0 wt % of the total of Li2O, Na2O and K2O.
JP-A-61-242926 discloses a glass for bonding ferrite, containing SiO2, BaO, B2O3, Al2O3 and ZnO.
JP-A-2-97435 discloses a glass sealant for a high temperature thermistor device.
Each of JP-A-7-291656 and JP-A-9-268026 discloses an insulating glass composition containing Bi2O3.
In general, it is necessary to have a sophisticated melting technique for producing phosphate-based glass, vanadium-oxide-based glass, and antimony-oxide-based glass. Furthermore, components of these glasses tend to evaporate during the melting step, thereby making these glasses unstable and colored.
It is therefore an object of the present invention to provide a low-melting glass capable of easily providing a covering film that has a thermal expansion coefficient close to that of the substrate, a low softening point, superior dielectric characteristics, a superior transparency and a superior electric insulation.
According to the present invention, there is provided a transparent, electric insulating, low-melting glass for covering a substrate. This glass comprises chemical components of SiO2, B2O3, BaO and ZnO; a thermal expansion coefficient of from 65xc3x9710xe2x88x927/xc2x0 C. to 95xc3x9710xe2x88x927/xc2x0 C. within a range of 30-300xc2x0 C.; a softening point of not higher than 600xc2x0 C.; and a dielectric constant of not greater than 7.5 at a frequency of 1 MHz under room temperature.
A film of the low-melting glass according to the invention may directly cover the substrate (e.g., a display panel) or to cover an element disposed on the substrate. This element may be at least one of a conducting material and a semiconductor pattern, such as (a) a bus electrode pattern, for example, made of at least one of Cu and Ag, and (b) a transparent electrode pattern, for example, made of In2O3, SnO2 and In2O3 doped with Sn (ITO).
It is preferable that the glass comprises 0.1-10 wt % of said SiO2, 20-35 wt % of said B2O3, 25-55 wt % of said ZnO, and 15-50 wt % of a total of said BaO and RO representing at least one of MgO, CaO and SrO. Furthermore, it is preferable that the weight ratio of said BaO to the total of said BaO and said RO is 1:2 or greater.
According to one preferred embodiment of the present invention, there is provided a transparent, electric insulating, low-melting glass for covering a substrate. This glass comprises chemical components of SiO2, B2O3, BaO and ZnO; a thermal expansion coefficient of from 75xc3x9710xe2x88x927/xc2x0 C. to 85xc3x9710xe2x88x927/xc2x0 C. within a range of 30-300xc2x0 C.; a softening point of not higher than 600xc2x0 C.; and a dielectric constant of not greater than 7 at a frequency of 1 MHz under room temperature. This glass preferably comprises 2-10 wt % of said SiO2, 20-30 wt % of said B2O3, 34-43 wt % of said ZnO, and 30-37 wt % of said BaO. Furthermore, this glass is substantially free of PbO, Bi2O3 and an alkali metal oxide. This glass may further comprise not greater than 15 wt % of a component substituted for said BaO. This component is at least one of MgO, CaO and SrO.
The substrate, which is to be covered by the low-melting glass, may be a transparent glass substrate, such as a soda-lime-silica glass or a similar glass (high strain point glass) or an alumino-lime-borosilicate glass. The substrate may have a thermal expansion coefficient of from about 65xc3x9710xe2x88x927/xc2x0 C. to about 95xc3x9710xe2x88x927/xc2x0 C. within a range of 30-300xc2x0 C. The low-melting glass can have a thermal expansion coefficient close to that of the substrate. With this, it becomes possible to prevent exfoliation of the film from the substrate and/or warp of the substrate. The substrate may have a softening point of from about 720 to about 840xc2x0 C. In contrast with this, the low-melting glass has a softening point of not higher than 600xc2x0 C. Therefore, when the film is baked, it become possible to prevent softening, deformation and thermal contraction of the substrate.
In general, an insulating film (low-melting glass) formed on a display substrate is desired to have a lower dielectric constant. In particular, a low-melting glass of PDP is desired to have a lower dielectric constant (e.g., not greater than 9) in order to reduce the electric power consumption for driving PDP. According to the invention, the dielectric constant of the low-melting glass is not greater than 7.5, preferably not greater than 7, at a frequency of 1 MHz under normal temperature (room temperature). Thus, the low-melting glass can contribute considerably to electric power saving of PDP.
As stated above, the low-melting glass comprises chemical components of SiO2, B2O3, BaO and ZnO such that the glass can be adjusted to having the above-mentioned numerical ranges of thermal expansion coefficient, softening point and dielectric constant. It is preferable that said glass is substantially free of each of PbO and Bi2O3. With this, the adverse impact of the low-melting glass on human and environment can substantially reduced. It is also preferable that the glass is substantially free of alkali metal oxide. With this, the film of the low-melting glass becomes superior in electric insulation. Furthermore, it becomes possible to prevent damage caused by the alkali accumulation on the surface of the film.
As mentioned above, it is possible to directly cover a substrate with the film of the low-melting glass, for example, in order to prevent alkali migration from an alkali-containing glass substrate or to alter optical characteristics of the substrate. Furthermore, it is possible to form at least one of various functional films on the film of the low-melting glass. It is possible to prepare a frosted glass for reducing glare of the solar radiation and artificial illumination, by mixing a low-melting glass powder with a silica fine powder, an alumina fine powder and/or the like and by forming a film from the resulting mixture on a glass substrate.
The description of chemical components of the low-melting glass is as follows. SiO2 is a glass forming component. It is possible to make a stable glass even with a small amount of SiO2, if it is coexistent with another glass forming component of B2O3. The SiO2 content of the glass is preferably 0.1-10 wt %. If it is less than 0.1 wt %, the glass may become unstable. If it is greater than 10 wt %, the softening point of the glass may become too high, making formability and workability difficult. A PDP can be prepared by forming an insulating film to cover an electrode pattern and then by removing a portion of the insulating film with acid to expose a portion of the electrode pattern. If the SiO2 content of the glass is excessive, the glass may become too high in acid resistance. Thus, it is more preferably not greater than 5 wt % in order to smoothly decompose the film with acid. It is more preferably not less than 2 wt % in order to make the glass more stable. It is optional to replace half or less by weight of SiO2 with Al2O3 to an extent that the Al2O3 content of the glass is not greater than 1 wt %.
Of the chemical components of the glass, B2O3 is a glass forming component similar to SiO2. B2O3 makes it easy to melt the glass, suppresses the excessive increase of the thermal expansion coefficient of the glass, and provides the glass with a suitable fluidity upon baking. Furthermore, B2O3, together with SiO2, serves to lower the dielectric constant of the glass. The B2O3 content of the glass is preferably 20-35 wt %, more preferably 22-30 wt %. If it is less than 20 wt %, the glass may become unstable. Thus, devitrification tends to occur. If it exceeds 35 wt %, the softening point of the glass may become too high.
ZnO has functions of lowering the softening point of the glass, providing the glass with a suitable fluidity and adjusting the glass to having a suitable thermal expansion coefficient. The ZnO content of the glass is preferably 25-55 wt %, more preferably 34-54 wt %. If it is less than 25 wt %, these functions may become insufficient. If it is greater than 55 wt %, the glass may become unstable. Thus, devitrification tends to occur.
Similar to ZnO, BaO also has functions of lowering the softening point of the glass, providing the glass with a suitable fluidity and adjusting the glass to having a suitable thermal expansion coefficient. Similar bivalent metal oxides, MgO, CaO and SrO, also have functions of providing the glass with a suitable fluidity and adjusting the glass to having a suitable thermal expansion coefficient. The glass preferably comprises 15-50 wt % of the total of BaO and RO representing at least one of MgO, CaO and SrO. If it is less than 15 wt %, the above functions may become insufficient. If it is greater than 50 wt %, the thermal expansion coefficient may become too high. The weight ratio of BaO to the total of BaO and RO is preferably 1:2 or greater. If this ratio is less than 1:2, devitrification tends to occur, or the softening point may become too high, or the thermal expansion coefficient may deviate from a suitable range. The BaO content of the glass is preferably 16-35 wt %.
The glass may contain PbO, Bi2O3 and alkali metal oxide as impurities, so long as the amounts of these impurities are each not greater than 0.3 wt %. With this, the above-mentioned adverse effects on human, environment and insulating property can be negligible.
In case of directly cover a substrate with a film of the low-melting glass, the glass may contain additives of Fe2O3, Cr2O3, CoO and CeO2 and the like for providing the glass with a certain color, ultraviolet absorption capability, infrared shielding capability and the like, so long as these additives do not interfere with the glass to have the above-mentioned thermal expansion coefficient, softening point and dielectric constant. The total of the above additives is preferably not greater than 1 wt %.
In case that the film of the low-melting glass covers a transparent electrode pattern that is made of an oxide (e.g., at least one of In2O3, SnO2 and In2O3 doped with Sn (ITO)) and disposed on a substrate of a display panel (e.g., PDP), it is preferable that the low-melting glass also contains 0.5-5 wt % of this oxide so long as the oxide does not interfere with the glass to have the above-mentioned thermal expansion coefficient, softening point and dielectric constant. Similarly, in case that the film covers a bus electrode pattern that is made of a metal (e.g., at least one of Cu and Ag) and disposed on a substrate of a display panel (e.g., PDP), it is preferable that the low-melting glass also contains 0.1-1.5 wt % of an oxide (e.g., at least one of CuO and Ag2O) of this metal. Due to the above containment of at least one of these oxides, it becomes possible to effectively suppress both of (1) a first corrosion of the low-melting glass by the transparent electrode pattern and/or the bus electrode pattern and (2) a second corrosion of the transparent electrode pattern and/or the bus electrode pattern by the low-melting glass. In particular, if the low-melting glass contains 0.1-1.5 wt % of CuO, the low-melting glass is provided with an advantageous filter function that makes blue color of PDP clearer. In fact, PDP is generally inferior in blue color emission. Therefore, the containment of a suitable amount of CuO is particularly preferable. If the amount of at least one of the above oxides is too much, the low-melting glass may have inferior thermal characteristics and an undesirable color.
An exemplary process for producing a front substrate of a PDP will be described as follows. This front substrate is made of a clear soda-lime glass or another glass having a chemical composition, thermal characteristics and the like similar to those of clear soda-lime glass. A transparent electrode pattern, for example, made of an ITO-based or SnO2-based material is formed on a surface of the front substrate by sputtering or chemical vapor deposition (CVD). Furthermore, a bus electrode pattern, for example, made of Cu-based material is formed on the front substrate by sputtering or the like in a manner to be in contact with the transparent electrode pattern. Then, a transparent insulating film made of a low-melting glass according to the invention is formed on the surface of the front substrate in a manner to cover the front substrate, the transparent electrode pattern and the bus electrode pattern. In fact, this insulating film is prepared by mixing a low-melting glass powder, which has been adjusted to having a certain desired particle size, with a paste oil, then by applying the resulting mixture onto the front substrate, the transparent electrode pattern and the bus electrode pattern by screen printing or the like, and then by baking the resulting precursory film at about 600xc2x0 C. into an insulating film having a thickness of about 30 xcexcm. This thickness of about 30 xcexcm is considered to be a sufficient thickness to achieve display capability and display stability for a long time by gas discharge. Furthermore, a protective magnesia layer is formed on the insulating film by sputtering or the like, thereby completing the production of a PDP front substrate. In some cases, there is a necessity to remove a portion of the insulating film by applying an acid for exposing and connecting a portion of the electrode with an outside lead wire. In these cases, the insulating film preferably has a suitable acid solubility.